Multileaf collimators (MLCs) are widely used in radiation machines to support various radiation treatments. A multileaf collimator includes a plurality of beam shaping leaves independently movable in and out of a radiation beam to block or modify the beam. The beam shaping leaves are generally arranged in pairs and disposed in opposing banks. The combined positioning of all leaves may define one or more apertures through which an unblocked radiation beam passes. The one or more apertures may define a treatment field in the isocenter plane, with a size and/or shape generally conforming to the size and/or shape of a target located in the isocenter plane. A maximal treatment field is generally provided when all the beam shaping leaves are fully retracted.
FIG. 1 depicts a conventional radiation machine 10 including an MLC in conjunction with collimation jaws. The radiation machine 10 includes a radiation source 12, a primary collimator 14 and a secondary collimator 16 adjacent to the source 12, flattening filters 18, and an ion chamber 20. The radiation machine 10 also includes collimation jaws 22a and 22b that can be motorized and positioned to generally limit the size of the beam 24 from the source 12. An MLC 26, supported by a mounting plate or MLC interface plate 28, further collimates the beam in finer resolutions to provide a treatment field with a size and/or shape generally conforming to an intended target in the isocenter plane.
In the conventional radiation machine 10 shown in FIG. 1, a significant extra-focal dose 30 may appear in the proximity of an intended treatment field, especially when the intended treatment field comes near the edge of the maximal treatment field of the MLC 26 as the collimation jaws 22a and 22b are forced to open to a large field. The extra-focal leakage is caused in part by the radiation scatter through the gap between the collimation jaws e.g. Y-jaws 22b and the outermost beam shaping leaves of the MLC 26. As the size of an intended treatment field increases, the extra-focal dose or out-of-field dose (OOFD) 30 delivered to the patient also increases. A few scattered photon trajectories are schematically shown to originate from the flattening filter 18 which is the main source of scattered radiation. Additionally, scatter from the upper collimator 14, ion chamber 20, jaws and MLC leaves 26 also contribute to the extra-focal dose. FIG. 1A schematically shows an extra-focal leakage measurement outside a field with the MLC being closed on axis.
Extra-focal leakage may lead to considerable dose to surrounding critical organs and/or healthy tissue, creating higher secondary cancer risk. Therefore, it is desirable to develop a multileaf collimator assembly capable of preventing or significantly reducing extra-focal radiation leakage in the proximity of the outermost MLC leaves.